Thin reinforced concrete elements, for example flat concrete slabs, provide an elegant form of construction, which simplifies and speeds up site operations, allows easy and flexible partitioning of space and reduces the overall height of buildings. Reinforced concrete flat slab construction also provides large uninterrupted floor areas within a minimum construction depth, and is used extensively for a wide range of buildings such as office blocks, warehouses and car parks.
One design problem associated with this form of construction is punching failure, which occurs as a result of high point loads or high shear stresses around the supporting columns. In punching failure, the failed surface of the slab forms a truncated cone or pyramid. This problem has in the past often lead to the use of mushroom heads or local thickening of the slab, but these solutions increase costs and slow down the rate of construction. As the spans become larger and the slabs become thinner the increased stresses around the critical shear perimeter have created even greater problems for the structural engineer. A variety of design solutions have been proposed, of which the most commonly used are as follows:
1. Conventional shear reinforcement
This solution is very labour-intensive and requires extra work both in the design and on site.
2. Use of a larger column and/or a thicker concrete slab
These solutions increase the deadload of the building and reduce the available space.
3. Use of a column head
This requires more complicated formwork, slows down the rate of construction, and interferes with the installation of building services.
4. Use of slab drops
These are a modified form of column head.
Shear reinforcement, when required, is normally accomplished by providing reinforcing members either at an angle or laterally to the main flexural reinforcement. In thin structural elements, such as flat slabs, anchoring of short lengths of shear reinforcement is a major design problem. The problem is aggravated by the fact that normal shear reinforcement cannot be placed above the top layer of flexural reinforcement without reducing either the durability, or the efficiency, of the flexural reinforcement. In addition, there is the practical problem of supporting the shear reinforcement during the construction stages.
Recently a new system has been introduced by Square Grip Limited, designated the Shearhoop (registered trade mark) system, which consists of assembly of specially shaped links (shear leg bobs) and hoop reinforcing bars. The hoops are available in a range of sizes and can be combined to form a complete system extending outwards from the column to the zone where the shear resistance of the concrete slab alone is adequate.
In the construction of a slab using Shearhoop (RTM) hoops, bars B1, B2 for the bottom layer of reinforcement are first laid down and the hoops placed over them in the appropriate location. Top reinforcement T2 is then positioned on chairs and the bars overlapping the hoops fully located under the ends of the shear leg bobs extending from the hoops. Finally the top reinforcement T1 is placed over the entire structure.
Whilst the Shearhoop (RTM) system is an improvement on previous arrangements, the hoops still cannot be anchored above the top layer of reinforcement T1 and thus do not provide the best possible shear reinforcement.
From the above, it is apparent that, although much effort has gone into the design of reinforcing systems that address some of the above mentioned problems, none of them provide a complete solution. Although prepackaged reinforcing systems offer some time savings over the in-situ steel fixing solutions, they are nevertheless more expensive in terms of materials and other resources, such as labour and crane time. Some of the other prior art proposals are also of questionable effectiveness, or produce an unquantifiable increase in flexural capacity.
There is a need, therefore, for an improved reinforcing system to impart better shear resistance, without increasing the thickness of the slab. An additional advantage would be to provide a shear reinforcement system enabling thinner slabs to be used.
U.S. Pat. No. 4,854,106 describes foundations for buildings and like structures employing steel reinforcement. A hook leg has an elongate member bifurcated at each end longitudinally of the member to form a pair of extensions with a slot there between, the distal portion of the extensions being bent into a curved form extending transversely of the member to form hooks adapted to resiliently engage of pair of reinforcing rods in the reinforcement, the slots in the unbent portions of the extension being adapted to receive a second pair of reinforcing rods extending transversely of the first pair, whereby to fix the rods in spaced alignment. There is no mention of shear reinforcement.
U.S. Pat. No. 4,472,331 describes are inforcing framework for a concrete building structure in which column and beam reinforcing bars are inserted into holes in reinforcement frames disposed at predetermined intervals. Shearing reinforcement bands, formed by bending a steel strip into a rectangular frame shape, are disposed between adjacent reinforcement frames and secured to wooden sheathing boards by nails. The construction requires access to all sides of the column or beam, and the protruding nails would give rise to potential corrosion problems.
DE 3331276 describes shear reinforcement elements for column supported flat slabs or beams of reinforced or prestressed concrete, which consist of flat steel strips which are undulating in at least two dimensions and transverse to the main surface of the flat slab or beams. The shear reinforcement elements are used in place of conventional round reinforcing bars.
GB-A-292267 describes a method of securing top and bottom reinforcement cages in a road foundation where crossed rods from one cage are secured by a locking member arranged parallel to one of the rods and formed with a looped crutch into which the rods of the cage are threaded. The locking member then extends across to the parallel cage where a similar arrangement locks the rods of that cage together.